This invention relates to preventing downhole equipment from sticking in well boreholes. The invention comtemplates the use of improved drill collars and other well implements having a porous coating placed on at least a portion of those implements.
In the drilling of oil wells, gas wells, lixiviant injection wells, and other boreholes, various strata are bypassed in achieving the desired depth. Each of these sub-surface strata has associated with it physical parameters, e.g., porosity, liquid content, hardness, pressure, etc., which make the drilling art an ongoing challenge. Drilling through a stratum produces an amount of rubble and frictional heat; each of which must be removed if efficient drilling is to be maintained. In rotary drilling operations, heat and rock chips are removed by the use of a liquid known as drilling fluid or mud. Most rotary drilling apparatus use a hollow drill string made up of a number of drill pipe sections and, of course, a drill bit at the bottom. Drilling fluid is circulated down through the drill string, out through orifices in the drill bit where it picks up rock chips and heat and returns up the annular space between the drill string and the borehole wall to the surface. There it is sieved, reconstituted and directed back down into the drill string.
Drilling fluid may be as simple in composition as clear water or it may be a complicated mixture of clays, thickeners, dissolved inorganic components, and weighting agents.
The charactertistics of the drilled geologic strata and, to some extent, the drilling apparatus determine the physical parameters of the drilling fluid. For instance, while drilling through a high pressure layer, e.g., a gas formation, the density of the drilling fluid must be increased to the point that the hydraulic or hydrostatic head of the fluid is greater than the downhole pressure of the stratum to prevent gas leakage into the annular space surrounding the drill pipe and lower chances for a blowout.
In strata which are porous in nature and additionally have a low formation pressure, another problem occurs. Some of the drilling fluid, because of its hydrostatic head, migrates out into the porous layer rather than completing its circuit to the surface. One common solution of this problem is to use a drilling fluid which contains bentonite clay or other filtration control additives. The porous formation tends to filter the filtration control additive from the drilling fluid and form a filter cake on the borehole wall thereby preventing the outflow of drilling fluid. As long as this filter cake is intact, very little fluid is lost to the formation.
During drilling, the rotating drill string is closely adjacent or in contact with the filter cake. If the filter cake is soft, thick, or of poor quality or if the drill string thins the filter cake, then the higher hydrostatic head of the drilling fluid will tend to push the drill string into the filter cake. In some cases the drill string will stick to the borehole wall. This phenomenon is known as differential pressure or hydrostatic sticking. In severe cases, it will be impossible to either turn the drill string or even move it up and down the borehole. It is this problem for which the apparatus of the invention is a solution.
The two widely used methods of alleviating hydrostatic or differential pressure sticking attack the problem from different flanks; one is remedial and the other preventative.
Once a drill string is stuck against a filter cake adjacent a porous formation, the remedy of a chemical spotting agent is used. It is first necessary to determine where on the drill string the stickage has occurred. One such method involves stretching the drill string by pulling it at the surface. Charts are available correlating the resulting stretch (per amount of applied stress) with feet of drill pipe. Once this information is known, the injection of water-based drilling fluid is interrupted and the spotting agent substituted. The spotting agents are often oleophilic compositions and may be oil-based drilling fluids, invert emulsions of water in oil, or a material as readily available as diesel oil. After the slug of, typically, 10-50 barrels of spotting agent is introduced, addition of drilling fluid is re-commenced. The slug of spotting agent continues its trip down through the drill string, out the drill bit, and up the wellbore annulus until it reaches the site of the stickage. Upon arrival of the spotting agent at the stickage location, circulation is temporarily ceased. Those skillful in this art speculate that oil-based spotting agents tend to dehydrate the filter cake on the borehole wall and cause it to break up, thereby allowing the drill string to come free. In any event, once movement of the drill string is detected, circulation of the drilling fluid is restored. It should be observed that the cost of this process is high and the success rate only moderate.
A preventative method of allaying drill string stickage in porous formations entails the use of drill collars having flutes, spirals, or slots machined in the outer surfaces. This method is used to a lesser extent than the spotting agent method since it involves a higher capital expense, and results in lighter drill collars. Drill collars are, of course, used for the specific purpose of adding weight to the lower end of a drill string. Consequently, light drill collars are not viewed with much enthusiasm. Although these collars are somewhat more effective in preventing stickage, they are not immune to the problem since the exterior grooves can be plugged, inter alia, with soft clay.